The oxidation/corrosion resistance of silicon nitride is determined by a number of factors, most notably the amount and chemical composition of the grain boundary phases produced as a result of adding oxides to promote densification during sintering /hot pressing. The intergranular material may be very active during high temperature exposure, producing a deleterious effect on the oxidation/corrosion processes. A method for producing silicon nitride with low levels of additives (only 0.59% Al2O3 and 1.22% Y2O3) and the benefits in terms of corrosion behaviour are described in this paper. The results of tests in combustion gases, where the concentration of sulphur in the fuel and contaminants in the ingested air have particular effects on the rate of corrosion, are reported.